Method of electrolyzing brine



United States Patent IVIETHOD F ELECTROLYZING BRINE heodore W. Heiskell and Gordon A. Carlson, New

- Martinsville, and Shannon W. Brown, Glendale, W. Va., assignors to Columbia-Southern, Chemical Corporation, Allegheny County, Pa., a corporation of Delaware No Drawing. Filed July 11, 1957, Ser. No. 671,143

Claims. (Cl. 204-98) The present invention relates to improvements in the operation of electrolytic cells. More particularly, the

tions in electrolytic chlorine-caustic cells to effect increased anode current efficiency, to reduce graphite consumption and to reduce electric voltage.

Electrolytic cells of the type described in US. Patent vinstant discovery concerns the treatment of brine solul No. 1,866,065, which issued July 5, 1932, are typical of base or bottom, enclosing the anode-cathode system and In the spaces between the anodes ii) providing essentially two compartments, the anode compartment and the cathode compartment.

In operation, sodium chloride solution or brine to be subjected to electrolysis is fed through the top of the cell into the anode compartment and passes to the cathode compartment by virtue of the fact that the hollow cathode members have perforated walls generally covered with an asbestos diaphragm through which the electrolyte permeates. In the anode compartment, chloride gas is given up by the brine solution and passes upwardly and out of the cell through an opening provided in the cell cover. In the cathode compartment hydrogen is given up to the cathodic electrode and also passes from the cell, through an opening in one of .its walls, the remaining caustic-rich solution (cell liquor) being permitted to flow out of the cell through an opening near the bottom thereof.

In electrolytic cells of the type herein contemplated, there is a strong tendency for OH ions dissociated in the electrolyte solution to migrate from the catholyte compartment to the anolyte compartment as a result of electrical forces or by mere diffusion through the cath- "ode diaphragm. Probably the most serious problem occasioned by the migration of these OH" ions from the 'catholyteto the anolyte is that anode current efficiency is greatly reduced, based on the chlorine production per unit ofelectricity. In other words, any current loss to the production of oxygen or derivatives thereof, such as CO C10 and the like, inthe anolyte compartment as a result of the presence of OH- ions deprives the system p f Patented S p 1960- 2. of that much current which could be utilized for the production of chlorine.

On a commercial scale, therefore, the electrolysis of brine under optimum conditions requires efficient use of current at a given brine concentration and the production of an NaOH liquor product of'adequate concentration. 'Migration of OH- ions from the catholyte to the anolyte has in the past caused greatly reduced anode current efiiciencies. Obviously, all current expended in the formation of oxygen or lay-products thereof is lost current. The art has for years sought a solution to this very undesirable feature. These by-products occasion a further annoyance in that they contaminate the products and must, in many cases, be removed therefrom.

Another serious problem affecting the production of chlorine and alkali metal hydroxides in electrolytic cells of the type herein contemplated comprises the gradual chemical corrosion of the anode surfaces to the extent that they must be replaced with disturbing frequency; By corrosion the anode exposed to the electrolyte is gradually diminished in size or consumed and anode efliciencyis greatly reduced, the reduction being the result of diminished current-carrying ability on the part of the anode.

Furthermore, the cost resulting from frequent shutdowns of a cell, or at times complete cell circuits, to replace corroded anodes is very significant, not to mention the cost of the anodes themselves. Generally, these anodes are made of graphite (carbon) and, if shortlived, represent an expensive item in the production of chlorine.

Anode corrosion is commonly recognized as being caused by the reaction of oxygen with the carbon in the graphite anode to produce carbon dioxide. It can be seen, therefore, that in this particular case poor anode current, efiiciency has a doubly undesirable consequence. Dissipated current produces oxygen which, in turn, reacts With the carbon anode to produce carbon dioxide, thus accounting for the corrosion of the anode. These are equally costly and annoying drawbacks with which the art has had to cope for many years.

According to the present invention, however, these drawbacks have been significantly minimized. The instant discovery involves a method of preparing chlorine by the electrolysis of brine solutions in electrolytic cells of the type herein contemplated which comprises acidifying the brine withsufficient hydrochloric acid to provide 1 to 15 percent'by weight theoretical acid equivalent based on the theoretical chlorine-producing capacity of the cell per minute, preferably at least 4 percent, and raising the upper brine level in the cell to at least 3 inches abovethe top of the electrodes to maintain the levels as high as 2 0 inches or more may be used.

In order toaccomplish this increase in brine level, it

is generally desirable to increase the height of the sides of the cells proportionately. For example, where the overflow height of the upper brine level is about 10.5 inches above the tops of the electrodes in a cell of the type contemplated herein and the cell is normally operated, when no acid is added, at a brine level of about 6.5 inches above the electrodes, an increase of the normal upper brine level of about 5 inches to maintain a substantially constant flow of liquor through the cell upon acidification necessitates a proportionate increase in the height of the sides of the cell of at least one (1) inch to accommodate the raised upper brine level. For all practical purposes the sides are generally raised at least 3 inches above their normal overflow level, thus, of course, establishing a higher overflow level.

Convention-ally, an L-shaped sight glass tube open at each end is used as an overflow means and a brine level indicator. One end of the tube is fitted into one of the cell walls at a point intermediate the top level of the electrodes and the top of the wall and the'tube extends outwardly from the wall and upwardly to a height which is approximately the same as that of the wall. Being that the sight glass is in communication with the brine solution in the cell, the level of brine shown in the glass is the same as that in the cell.

The theoretical acid equivalent added to a cell, according to the instant discovery, can be computed by multiplying the faradays per minute delivered to a "given cell times the gram atomic weight of chlorine (35.457), the product being the theoretical weight of chlorine which should be produced per minute, basis the fact that one faraday is required to liberate one gram atomic weight of chlorine. Of course, when the cell is delivering only 90 percent of the chlorine it is theoretically possible for it to produce, it is said to be operating at 90 percent anode current efliciency.

Multiplication of the quantity of HCl added in grams per liter by the rate of flow in liters of HCl per minute times 36.465 (HCl) 4 liter HCl is fed at the rate of .04 liter per minute to the cell, the theoretical acid equivalent fed is computed as follows:

200 grams HCl per liter X .04 liter HCl solution per minute 35.457 (Ch atomic weight) or 1 36.465 (H Cl atomic weight) 1,028

=7.87 grams chlorine per minute 7.87 grams chlorine per'minute (HCl) X 100 286.49 grams chlorine per minute (brine theoretical) :27 percent theoretical equivalent (HCl) by weight While the chemical consequences of acid addition as contemplated herein are not fully understood, it is certain that the present discovery greatly enhances anode current efiiciency, substantially reduces graphite (car-- bon) anode consumption and reduces the voltage required clearly within the scope of the present invention are gives the theoretical amount of atomic chlorine in grams For example, for a cell at 13,000 amperes the amount (weight) of chlorine theoretically recoverable per minute is computed as follows:

13,000 amperes WX 8.08 faradays/mmute 8.08 35.457 (Cl gram atomic weight)=286.49 grams of chlorine per minute (theoretical) If it is determined by analysis that the cell is actually I producing only 257.84 grams of chlorine per minute,

then it is operating at or 90 percent anode efiiciency.

Now, if an HCl solution containing 200 grams per and will be apparent to the skilled chemical engineer; As to the concentration of HCl solution added, it is regulated by convenience only since the quantity required is not that great.

The present invention will best be understood by reference to the following detailed examples, but it is not intended that these examples exert undue restrictions upon the scope of the invention. It will be readily perceivable that innumerable variations are contemplated herein which do not transcend the spirit and breadth of the invention.

EXAMPLE I From a series oircuit'o-f 40 cells of the type hereinabove described, ten (10) cells were selected atrandom and provided with inlets in their covers for the introduction of hydrochloric acid solution, the inlets being above and near the brine feed opening.

These cells were converted from cells having overflow levels of 10.5 inches above the tops of the electrodes therein to cells having overflow levels of 14.5 inches above the electrode tops by raising the side wall heights by four inches.

Whereas these cells, prior to acidification, wereoperating at an average upper brine level about 6.46 inches above the electrode tops, acidification according to Table I increased this average upper level to about 10.5 inches above the tops of the electrodes.

Prior to acidification, the anolyte gases from each operating cell were analyzed for CO and, when it was determined that the CO concentration was substantially constant, the analysis was discontinued, the percent CO at the substantially constant value being then recorded. In addition, samples of the brine fed to' the cell, the electrolyte (anolyte) liquor and the causticrich cell liquor were taken for analysis and the cell voltage recorded. 1 I

Immediately thereafter, hydrochloric acid of twenty (20) percent concentration, which is a constant boiling solution, was fed at a controlled rate into the anolyte Moles NaCl per minute NaCl'fiow grams per liter NaCl in brine feed X3.785Xgallons per minute brine flow 58.454 (molecular Weight of NaCl) amperes 60 I Percent, NaCl cnvers1on 96,500

p 58454 X', grams per liter NaCl in brine f eed ifi c X3.785Xgallons per minute brine flow X100 Brine flow in gallons per minute 0.9602 amperesXinitial current efficiency grams per liter NaCl in brineXpercent conversion In run 1 the flow through the cell during acidification pursuant to the method taught by the present invention is as follows:

Percent conversion ii fil i%. 1 462 1564X 160.5X loo-58.86%

Flow in gallons per minute From these comparative data it is readily apparent that prior to acidification and. during acidification the flow of electrolyte (anolyte and catholyte) through the cell does not vary substantially, if any, when practicing the process of the present invention. By simple computation (as above) the flow rate of the remaining runs can be figured, what with data in the tables.

Although the foregoing examples teach the treatment of brine solutions containing NaCl, other alkali metal halides, such as KCl, are also contemplated. Furthermore, it is preferable to maintain an alkali metal equivalent in the anolyte solution of at least about 4.3 grams per liter to provide the benefits of the present invention.

Also, the upper brine level is generally maintained at least about 6 inches above the catholyte level in the catholyte compartment, preferably between about 11 and 15 inches, although an upper brine level as high as 23 inches or more above the catholyte level may be used.

It can be seen, therefore, that acidification according to the methods of the instant discovery affords highly beneficial results, such as increased anode current efliciency, substantially reduced graphite consumption and numerous other advantages which are readily apparent from the foregoing description.

While the present invention has been defined with respect to details of specific embodiments thereof, it is by no means limited thereto. It is obvious that numerous modifications are clearly within the spirit and breadth of the discovery.

We claim: V

1. In a method of preparing chlorine and caustic by electrolysis of a brine solution in an electrolytic diaphragm cell having an asbestos diaphragm and anormal operating brine level above the electrode tops of said cell the improvement comprising adding suflicient. hydro-j chloric acid to the brine solution to provide 1 to 15 per-1 cent byweight theoretical acid equivalent-based on the theoretical chlorine-producingcapacity of .the cellper minute while increasing and maintaining the brine level in the cell at least 3 inches higher than said normal level during the acid addition. V

2. .In a method of preparing chlorine and caustic by electrolysis of a brine solution in an electrolytic diaphragm cell havingan asbestos diaphragm and a normal operating brine level above the electrode tops of said cell the improvement comprising adding sufiicient hydrochloric acid to the brine solution to provide 1 to15 percent by weight theoretical acid equivalent based on the theoretical chlorine-producing capacity of the cell per minute, while maintaining the brine level in the cell between 8 and 12 inches above the electrode tops.

3. In a method of preparing chlorine and caustic by electrolysis of a brine solution in an electrolytic dia phragm cell having an asbestos diaphragm and a normal operating brine level above the electrode tops of said cell the improvement com-prising adding sufiicient hydrochloric acid to the brine solution to provide 1 to 15 per cent by weight theoretical acid equivalent based on the theoretical chlorine-producing capacity of the .cell per minute while increasing and maintaining the brine level of the cell between 3 and 23 inches higher than said normal level during the acid addition.

4. In a method of preparing chlorine and caustic by electrolysis of a brine solution in an electrolytic diaphragm cell having an asbestos diaphragm and a normal operating brine level above the electrode tops of said cell the improvement comprising adding sufiicient hydrochloric acid to the brine solution to provide 1 to 15 percent by weight theoretical acid equivalent based on the theoretical chlorine-producing capacity of the cell per minute while maintaining the brine level in the cell between 8 and 20 inches above the electrode tops during said acid addition.

5. In a method of preparing chlorine and caustic by electrolysis of a brine solution in an electrolytic diaphragm cell having an asbestos diaphragm and a normal brine level above the electrode tops of said cell under normal operating conditions the improvement comprising adding sufficient hydrochloric acid to the brine solution to provide 1 to 15. percent by weight theoretical acid equivalent based on thetheoretical chlorine-producing capacity of the cell per minute while increasing and maintaining the brine level in the cell between 3 inches and 20 inches higher than said normal level during the acid addition. 1

Great Britain of 1898 

1. IN A METHOD OF PREPARING CHLORINE AND CAUSTIC BY ELECTROLYSIS OF A BRINE SOLUTION IN AN ELECTROLYTIC DIA PHRAGM CELL HAVING AN ASBESTOS DIAPHRAGM AND A NORMAL OPERATING BRINE LEVEL ABOVE THE ELECTRODE TOPS OF SAID CELL THE IMPROVEMENT COMPRISING ADDING SUFFICIENT HYDROCHLORIC ACID TO THE BRINE SOLUTION TO PROVIDE 1 TO 15 PERCENT BY WEIGHT THEORETICAL ACID EQUIPMENT BASED ON THE THEORETICAL CHLORINE-PRODUCING CAPACITY OF THE CELL PER MINUTE WHILE INCREASING AND MAINTAINING THE BRINE LEVEL IN THE CELL AT LEAST 3 INCHES HIGHER THAN SAID NORMAL LEVEL DURING THE ACID ADDITION. 